This invention generally relates to digital data communications networks and more specifically to diagnostic circuits used in those networks.
A typical digital communications network includes a central site and one or more remote sites, or "drops." Each such remote site connects to the central site by means of telephone or other equivalent communications links. Typically there is a modulator-demodulator (i.e., a modem) at each of the central and remote sites.
The aforementioned U.S. application Ser. No. 503,625 discloses diagnostic circuitry that is used in such digital data communications networks. This circuitry connects to the modem of each site and allows an operator at a central site to identify the diagnostic circuitry at any one of several remote sites, and then to test the corresponding modem. Such a modem is a "host" modem and the diagnostic circuitry is used to ascertain the operative status of the host modem, in many cases without interrupting normal communications over the communications link. This circuitry, however, is limited to a network in which there are no intervening modems between the central site and each designated remote site.
In some networks, however, the remote sites are widely dispersed geographically. Yet within such networks, several remote sites might be clustered geographically. A digital data communications network, to which the system described in the forementioned Ser. No. 503,625 is applied, requires redundant telephone lines between the various sites. In a typical application, for example, a central site might be located in New York City and one or more groups of remote sites located in Cleveland and Akron, Ohio. Such a system requires two telephone lines. One telephone line extends from New York City to Cleveland and the other telephone line extends from New York City to Akron. The two New York-to-Cleveland telephone lines are redundant.
Therefore, digital data communications networks now interpose a "hubbing" site, when possible, to eliminate the redundant telephone lines. For example, Cleveland could be selected as a hubbing site, and therefore, the system would require only one telephone line between New York City and Cleveland. Then one line would connect to the remote sites in Cleveland and another line would extend from the Cleveland hubbing site to all the remote sites in Akron. Such a hubbing site usually contains either a time-division or frequency-division multiplexer and demultiplexer to properly switch communications between the single line from New York City and the two lines from Cleveland and Akron.
Due to the characteristics of various multiplexers, time-division multiplexing is preferred in these systems. Time-division multiplexing allows greater amounts of data to pass through the hubbing site. However, this method requires binary signals, so it is necessary to convert the incoming analog signals into binary signals and then to reconvert the binary signals into analog signals as data passes through the hubbing site. Even so, time-division multiplexers are more simple to construct and operate than are frequency-division multiplexers.
As the diagnostic circuitry described in the foregoing application requires that the remote site and central testing unit be coupled by a "continuous" analog path, e.g., a telephone line, this diagnostic circuitry can not be used in these new types of digital data communications networks. Basically, the conversion to a digital form at each hubbing site constitutes a digital barrier through which analog testing messages cannot pass.
In other typical systems, the digital data communications network may include multiple computers and their respective communications facilities for connection to various terminals. In such networks it is desirable to diagnose problems in any individual computer or any of its connected terminals from a single site. Again, the foregoing diagnostic circuitry in Ser. No. 503,625 will not operate in such a mode because the analog testing message encounters a digital barrier.
Therefore, it is an object of this invention to provide diagnostic circuitry which can be used flexibly in conjunction with various types of digital data communications networks.
Another object of this invention is to provide a diagnostic test circuitry that is operable accross digital barriers.
Still another object of this invention is to provide diagnostic circuitry which is compatible with prior diagnostic circuitry.